1. Field of the Invention
The present invention relates to a conductive paste for forming terminal electrodes of multilayer ceramic electronic parts such as multilayer ceramic capacitors, multilayer ceramic inductors, laminated type piezoelectric elements and the like. In particular, the present invention relates to a conductive copper paste suitable for forming terminal electrodes of multilayer ceramic electronic parts that have base metal internal electrodes made of nickel or the like.
2. Description of the Prior Art
Multilayer ceramic electronic parts, e. g., multilayer ceramic capacitors, are generally manufactured as follows. An internal electrode conductive paste is printed in a specified pattern on a dielectric ceramic green sheet such as a barium titanate ceramic or the like. Several individual sheets of this type are laminated, and are pressed together, thus producing an unfired laminate in which ceramic green sheet layers and internal electrode paste layers are alternately laminated. The laminate thus obtained is cut into chips of a specified shape, and these chips are then co-fired at a high temperature, thus producing multilayer ceramic capacitor bodies. Next, the end surfaces of each ceramic capacitor body on which the internal electrodes are exposed are coated by dipping or the like with a terminal electrode conductive paste consisting chiefly of a conductive powder, a glass powder and an organic vehicle; then, following drying, terminal electrodes are formed by firing at a high temperature. Subsequently, a plating layer of nickel, tin or the like is formed (if necessary) on top of the terminal electrodes by electroplating or the like.
Conventionally, noble metals such as palladium, silver-palladium, platinum and the like have been used as internal electrode materials. However, because of requirements in terms of resource conservation, cost reduction and prevention of delamination and cracking caused by oxidation expansion during the firing of palladium or silver-palladium, base metals such as nickel, cobalt, copper and the like now constitute the mainstream of such materials. Accordingly, copper, nickel, cobalt or alloys of these metals which allow the ready formation of good electrical connections with base metal internal electrodes are also used instead of silver or silver-palladium as terminal electrode materials.
In cases where base metals are thus used in internal electrodes and terminal electrodes, the firing of the terminal electrodes is ordinarily performed at a maximum temperature of 700 to 900° C. in a non-oxidizing atmosphere with an extremely low oxygen partial pressure, e. g., an inert gas atmosphere containing several ppm to several tens of ppm of oxygen.
However, in cases where (in particular) a terminal electrode conductive paste consisting chiefly of copper is fired in such an atmosphere containing little oxygen, organic components such as binder resins, solvents and the like used as vehicles tend not to be decomposed by oxidation; accordingly, the appropriate combustion, decomposition and removal (also called “binder removal”) of the organic components is difficult to perform in an appropriate manner. Specifically, in the initial stage of firing which is performed at a relatively low temperature, unless binder removal is sufficiently performed prior to the occurrence of the fluidization of the glass and sintering of the copper powder, a carbon and carbonaceous organic residues such as vehicle decomposition products or the like will be enclosed in the film following the initiation of sintering. Such enclosed carbon and carbonaceous organic residues (hereafter referred to as “residual carbon” in some cases) leads to various problems in subsequent high-temperature stages, causing a loss of characteristics by the electronic parts, and lowering the reliability of such parts. For example, in the stage in which the copper powder is sintered at a high temperature, carbon remaining in the film impedes the fluidization of the glass and sintering of the copper, so that the fine dense texture of the electrode and adhesion to the ceramic body are impaired. Furthermore, such residual carbon captures oxygen from the dielectric ceramic, thus creating an oxygen deficiency so that the dielectric characteristics are caused to deteriorate, and so that the strength of the ceramic body is also caused to drop. As a result of such a drop in the strength of the ceramic body, cracking of the ceramic body caused by thermal shock (thermal cracking) occurs in the subsequent soldering process or the like. Furthermore, when the sealed-in residual carbon is converted into a gas at high temperatures, blisters (bubbles) are formed so that the texture of the sintered film is impaired. As a result, when a plating treatment is subsequently performed on the sintered film, the plating solution penetrates into the electrode film, causing a drop in the insulation resistance and cracking of the ceramic body; furthermore, the penetrating plating solution is heated during solder reflow and converted into a gas, thus leading to “solder sputtering”, which cause the molten solder to scatter.
Accordingly, the question of how to efficiently achieve binder removal in the initial stage of firing, so that the residual carbon can be reduced prior to the sintering of the copper powder in a high-temperature region, has been an important problem in terminal electrode conductive pastes that consist chiefly of base metals (especially copper).
Conventionally, in order to solve this problem, methods using a resin with good thermal decomposition characteristics such as an acrylic resin or the like as the binder resin, or methods using a glass having characteristics which are such that the glass tends not to soften at low temperatures, but rather softens following the removal of the vehicle, so that the texture of the electrode is made finer, have been employed.
Furthermore, terminal electrode pastes using a fine spherical copper powder form excessively dense films when applied and dried; accordingly, it appears that the vehicle tends not to be driven off, and that carbon remains up to high temperatures. Accordingly, the use of a flaky copper powder instead of a spherical copper powder has been proposed. For example, Japanese Patent Publication No. 8-180731A discloses a multilayer ceramic capacitor terminal electrode paste that contains a flaky copper powder, a spherical copper powder, a glass powder and an organic vehicle. Such a flaky copper powder creates appropriate spaces in the dried film of the paste; it appears that these spaces act as gas venting paths, so that binder removal can be smoothly performed in structural terms. Furthermore, in Japanese Patent Publication No. 2002-56717A, it is indicated that the binder removal characteristics can be improved without sacrificing the paste applying characteristics and fine texture of the film by setting the dry film density of the paste in a specified range.
Meanwhile, in order to perform binder removal efficiently, there are also methods in which the oxidative decomposition of organic material is accelerated by raising the oxygen concentration to several hundred ppm or higher in a temperature region of (for example) approximately 200 to 600° C. prior to the densification of the electrodes in the temperature elevation process at the time of firing, and then lowering the oxygen concentration and performing firing. For example, Japanese Patent Publication No. 10-330802A and Japanese Patent Publication No. 2001-338831A disclose methods in which a fine spherical copper powder is covered with an oxidation-resistant coating consisting of glass or the like, the binder removal process is performed in an atmosphere with a high oxygen partial pressure such as the atmosphere or the like, so that the vehicle is decomposed while preventing oxidation of the copper, and firing is then performed after lowering the oxygen partial pressure.
In recent years, there have been increasingly strict requirements for increased capacity, improved performance and improved reliability in multilayer ceramic electronic parts. Especially in the case of small-size large-capacity multilayer ceramic capacitors, the spacing between the internal electrodes is narrow, i. e., 1 to 2 μm, so that a defective capacitance tends to be generated unless the terminal electrodes are dense and finely textured. Accordingly, there is a need to perform binder removal more smoothly, and to form a denser and more finely textured final fired film that is free of oxidation. However, in cases where the terminal electrode conductive paste used is a copper conductive paste, reduction of the amount of residual carbon (excellent binder removal characteristics) and prevention of the oxidation of the copper are mutually conflicting objects which are such that one of these characteristics deteriorates if an attempt is made to improve the other, and if either of these characteristics is poor, a good electrode cannot be formed. In addition, the effect of residual carbon on the sintering of copper is also great, so that it is extremely difficult to satisfy the above-mentioned requirements regardless of the conventional method that is used.
For example, in the case of methods in which binder removal is performed as described above in an atmosphere with a high oxygen partial pressure after covering the surfaces of the copper powder with a glass coating, and firing is then performed after lowering the oxygen partial pressure, the anti-oxidation effect and binder removal characteristics in the low-temperature region are superior; however, atmosphere adjustment in the high-temperature region is difficult, so that it is difficult to perform firing without ultimately oxidizing the copper powder.
Conversely, however, in cases where binder removal is performed in a low-oxygen atmosphere with an oxygen partial pressure of several tens of ppm or less, the binder removal at low temperatures tends to be incomplete even if the film is formed with a structure that allows easy gas venting by using a flaky copper powder. This tendency is especially conspicuous in cases where the oxygen concentration in the firing atmosphere is a few ppm or less, or in cases where the number of chips simultaneously fired is large. Furthermore, in order to improve dispersion and prevent oxidation, flaky metal powders are ordinarily subjected to a surface treatment with fatty acid or its metal salt such as stearic acid or the like; however, according to research conducted by the present inventors, the presence of such substances contributes to blistering and deterioration of the ceramic body.
Furthermore, in the case of firing in such a low-oxygen atmosphere, it is extremely difficult to achieve strict control of the oxygen partial pressure to a value on the order of ppm, and to maintain the atmosphere at a fixed concentration. Specifically, when organic material contained in the paste decomposes, oxygen is captured from the atmosphere so that a reducing atmosphere is created, and metal oxidation-reduction occurs, so that the oxygen concentration shows a slight variation. Accordingly, the ease of binder removal and the degree of oxidation of the copper also vary according to slight differences in the ceramic body, the size of the chips, the quantity of chips fired at the same time, the organic composition in the paste, and the firing conditions such as the oxygen concentration, peak temperature, temperature profile and the like. Furthermore, since the amount of carbon that remains up to the high-temperature region in the firing process varies greatly according to the number of chips fired at the same time and the shape of these chips, it is especially difficult to obtain a stable local oxygen partial pressure in the vicinity of the bodies being fired. As a result, the characteristics fluctuate greatly so that there is considerable dispersion in the results.
However, according to electronic part standards and makers, there are currently differences in the types of ceramics and firing conditions used, so that there is a need for a paste with a wide process window which makes it possible to obtain terminal electrodes with superior characteristics in a stable manner under various firing conditions.